Chemical-mechanical polishing methods

ABSTRACT

A chemical-mechanical polishing (CMP) method includes applying a solid abrasive material to a substrate, polishing the substrate, flocculating at least a portion of the abrasive material, and removing at least a majority portion of the flocculated portion from the substrate. Applying solid abrasive material can include applying a CMP slurry or a polishing pad comprising abrasive material. Such a method can further include applying a surfactant comprising material to the substrate to assist in effectuating flocculation of the abrasive material. Such surfactant comprising material may be cationic which includes, for example, a quaternary ammonium substituted salt. Also, for example, the surfactant comprising material may be applied during polishing, brush scrubbing, pressure spraying, or buffing.

RELATED PATENT DATA

This patent resulted from a continuation application of U.S. patentapplication Ser. No. 09/990,706, filed on Nov. 20, 2001 now U.S. Pat.No. 6,835,121 which resulted from a divisional application of U.S.patent application Ser. No. 09/475,545, filed on Dec. 30, 1999 now U.S.Pat. No. 6,375,548.

TECHNICAL FIELD

This invention relates to chemical-mechanical polishing methods.

BACKGROUND OF THE INVENTION

Chemical-mechanical polishing (CMP), also known as chemical-mechanicalplanarization, is widely used in a variety of industries, including thesemiconductor processing industry. CMP can remove unwanted material froma substrate, planarize a substrate, and/or create a desired finish on asubstrate. All of such intentions may be generically termed “polishing.”Generally, the technology involves pressing some sort of solid abrasivematerial against the substrate to accomplish the polishing and/orplanarization. The solid abrasive material may be applied in a CMPslurry of such material and liquid carriers and/or chemically activecomponents as desired. Alternatively, abrasive material may be carriedwithin a polishing pad. Still other techniques are encompassed withinthe technology.

One common byproduct of CMP is that abrasive material residues oftenremain on the substrate. In some applications, residual abrasivematerial can negatively influence subsequent processing and/or result indefective products. Accordingly, a variety of approaches have beenattempted to resolve the problem of residual abrasive material.

One conventional approach is to use hydrofluoric acid-based chemistriesto undercut particles attached to a silicon oxide substrate. A problemwith hydrofluoric acid-based chemistries is that microscratches formedin the substrate as a result of CMP may be aggravated in the acidicconditions. Further, insoluble fluoride compounds may be formed fromreactions of hydrofluoric acid with the abrasive material.

Another conventional approach includes application of ammonium hydroxideor tetramethylammonium hydroxide (TMAH) to disperse residual abrasivematerial. At a high pH, a silicon oxide surface and most abrasivematerial particles, including ceria, alumina, and silica exhibit anegative surface charge. Such charge characteristics provideelectrostatic repulsion. Experimentally, such a method has producedlimited benefits and appears to work much better for aluminum oxideparticles in comparison to cerium oxide particles.

Still another conventional technique involves etching and/or dissolutionof abrasive particles. For cerium oxide particles, such may beaccomplished with the application of a mixture of hydrogen peroxide andsulfuric acid. While this method exhibits some effectivenessexperimentally, it is incompatible with any surface structures featuringexposed metal.

Accordingly, it is desired to provide a new method for removing CMPresidual abrasive material from a substrate.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, a chemical-mechanicalpolishing (CMP) method includes applying a solid abrasive material to asubstrate, polishing the substrate, flocculating at least a portion ofthe abrasive material, and removing at least a majority portion of theflocculated portion from the substrate. Such a method can includepolishing with a CMP slurry or polishing pad. It may further includeapplying a surfactant-comprising material to the substrate to assist ineffectuating flocculation of the abrasive material. Such surfactantcomprising material may be cationic which includes, for example, aquaternary ammonium substituted salt. Also, for example, thesurfactant-comprising material may be applied during polishing, brushscrubbing, pressure spraying, or buffing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

One aspect of the present invention provides a chemical-mechanicalpolishing (CMP) method which involves applying a solid abrasive materialto a substrate. Such substrate can include a semiconductor substrate. Inthe context of this document, the term “semiconductor substrate” or“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove.

Applying a solid abrasive material can include applying a CMP slurry ofsubstantially dispersed, solid abrasive material or applying a polishingpad comprising solid abrasive material. A variety of CMP slurries andpolishing pads are conceivable that can include a variety of components.Examples of solid abrasive material include cerium oxide, aluminumoxide, mixtures thereof, and combinations thereof with other materials.Preferably, the solid abrasive material comprises ceria. Accordingly,ceria-based abrasive material is also preferred. The solid abrasivematerial may be substantially dispersed within the CMP slurry, that is,the solid abrasive material is not entirely agglomerated into flocculecomprising multiple solid abrasive particles. Depending on theapplication, some agglomeration of abrasive material particles may betolerated. However, substantial dispersion alleviates the problem ofunnecessary scratching or other similar damage to a substrate fromoversized floccule.

The method next comprises polishing the substrate. The parameters underwhich such polishing is to occur may be established according to theknowledge of those skilled in the technology at the time the method isbeing practiced. That is, it is contemplated that the present method isapplicable both to currently available CMP parameters as well as othersthat may be later developed.

The method further includes flocculating at least a portion of theabrasive material on the substrate. Such flocculating may occur by avariety of means and at a variety of points within the CMP method. Onemeans for flocculating abrasive material includes applying a surfactantcomprising material. A variety of surfactant comprising materials aresuitable and may be characterized in a variety of ways.

One such suitable surfactant comprising material exhibits thecharacteristic of decreasing a settling time for the abrasive materialin an aqueous dilution of the slurry. The CMP slurry may be diluted inwater, such as de-ionized water, to produce an aqueous dilution of theCMP slurry having a desired concentration of the CMP slurry. Forexample, the aqueous dilution may comprise 0.1 weight percent (wt %) CMPslurry, 1 wt % slurry, or some other dilution level. The aqueousdilution of the CMP slurry will exhibit a settling time. That is, aftera desired amount of time passes, analysis can be conducted to determinethe extent to which solid abrasive particles have settled within theaqueous dilution. For example, analysis could occur on 24-hour cycles,or some other duration.

Settling time may vary depending upon a variety of factors, includingthe dilution level of the CMP slurry (i.e. initial particleconcentration in the dilution), pH, and the temperature of the aqueousdilution. It is expected that the most significant decreases in settlingtime compared to an aqueous dilution without a surfactant comprisingmaterial will occur when the temperature of the aqueous dilution doesnot exceed about 40° Celsius (° C.). A variety of settling times mayalso be used to measure the effectiveness of a surfactant comprisingmaterial. One example of a settling time is the elapsed time beginningfrom the mixing of a surfactant comprising material with the aqueousdilution up to the time when a designated percentage of the abrasivematerial has settled from the supernatant of the aqueous dilution.Settling time is considered to decrease if such elapsed time is less fora dilution with the surfactant comprising material.

Another way to characterize settling time is to compare the percentageof abrasive material that has settled from an aqueous dilution of theslurry after a set amount of time, for example, 24 hours. Settling timeafter addition of a surfactant comprising material is considered todecrease if an increased percentage of abrasive material settles fromthe supernatant in an aqueous dilution after a designated amount of timepasses.

In another aspect of the present invention, a suitable surfactantcomprising material exhibits a 1-hour settling rate constant of greaterthan 0.035 for the abrasive material in an aqueous mixture of about 0.1wt % surfactant and about 1 wt % CMP slurry containing 3 wt % abrasivematerial. Alternatively, the settling rate constant may be greater thanabout 0.09. As indicated above in the discussion on settling time, itmay be that the desired 1-hour settling rate constant is achieved whenthe temperature of the aqueous mixture does not exceed about 40° C.Settling rate constant may be calculated using the following equation:

$k = {\frac{1}{t}{\ln\left( \frac{c_{0}}{c_{t}} \right)}}$Wherein k is the settling rate constant, t is elapsed time, c₀ is theinitial concentration, and c_(t) is concentration at elapsed time t.Accordingly, in calculating the 1-hour settling rate constant, t=1 hour,c₀=initial concentration of abrasive material in the supernatant of theabout 1 wt % slurry dilution and c_(t)=the concentration of abrasivematerial in the aqueous dilution after one hour.

The weight percent of abrasive material in the CMP slurry may be knownand initial concentration can be calculated given the dilution comprisesabout 1 wt % slurry. However, improved accuracy in determining settlingrate constant is expected when initial concentration is determined byanalysis rather than by calculation. Inductively coupled plasma opticalemission spectrometry (ICP-OES) is one suitable analysis technique.Other techniques may also be used, such as gravimetric standard methodsfor determination of percent solids, however, they may be lesspreferable. Using ICP-OES, the weight percent of a particular metal,such as aluminum or cerium, is determined from the analytical techniqueand may often be equated with a concentration of abrasive material at aparticular time. That is, instead of determining the actualconcentration of abrasive material as a whole, the concentration of atracer material, such as aluminum or cerium, may be determined.Generally, a decrease in the concentration of the tracer material willbe proportional to a decrease in the concentration of the abrasivematerial.

In another aspect of the present invention, a suitable surfactantcomprising material may be characterized by its inclusion of particularsurfactants. For example, the surfactant may be cationic. Cationicsurfactants that are particularly suitable include quaternary ammoniumsubstitute salts, such as a quaternary ammonium halide. Specificsuitable surfactants include cetyltrimethylammonium bromide (availableas Rhodaquat-242B/99 from Ashland Chemical Co. in Dublin, Ohio) andpolyethoxylated quaternary ammonium halide. Examples of polyethoxylatedquaternary ammonium halide compounds include ethoxylated stearyl methylquaternary ammonium chloride and ethoxylated cocoalkyl methyl quaternaryammonium chloride. The two compounds are available as, respectively,Ethoquad 18/25 and Ethoquad C/25 from Akzo Nobel Surface Chemistry Inc.in Stratford, Conn. Of course, given the variety of ways in which asuitable surfactant may be characterized, it is expected that othercompounds may also be suitable. The concentration of the surfactant inthe surfactant comprising material can be, for example, about 10micrograms per milliliter (μg/ml) to about 10,000 μg/ml. Alternatively,the surfactant concentration can comprise about 100 μg/ml to about 1,000μg/ml.

As indicated, the flocculation may occur under a variety of conditions,but preferably when the temperature of the substrate does not exceedabout 40° C. Such temperature limit has been approximated as a pointbelow which improved flocculation is expected. Nevertheless, it is alsocontemplated that such temperature limit may vary with respect to aparticular surfactant, abrasive material, and/or substrate, among otherfactors. The flocculating can further comprise complexing at least aportion of the abrasive material with a surfactant. Such complexing mayin turn form floccule. The formation of floccule is one mechanismcontemplated by the present invention by which settling time may bedecreased and an appropriate one-hour settling rate constant may beachieved.

As suggested earlier, the flocculating can occur after the polishing. Inone aspect of the present invention, it is contemplated that primarypolishing of the substrate may be followed by buffing the substratealong with applying a surfactant comprising material and flocculating atleast a portion of the abrasive material. Primary polishing can includepolishing with a CMP slurry or a polishing pad comprising solid abrasivematerial. Buffing can be less aggressive, that is, use a softerpolishing pad, abrasive material that is less abrasive, and/or lesschemically active polishing media. Buffing may occur on a secondaryplaten of a CMP tool as opposed to a primary platen where primarypolishing often occurs. In some contexts, buffing can be considered partof polishing. Thus, flocculating can also occur during polishing.

One potential concern of applying surfactant comprising material duringa polishing and/or buffing step is that flocculated particles mayproduce undesirable scratches or other defects in a substrate. Thetendency for such scratches and/or defects to form tends to decrease asthe down-force of a polishing surface on a substrate is reduced. Often,the down-force of a polishing surface is less during a buffing step thanduring a primary polishing step. Accordingly, applying a surfactantcomprising material can be conducted during a buffing step, and perhapsanother low down-force or lesser aggressive polishing step. Of course,the ability to apply surfactant comprising material during such stepsmay be influenced by the hardness of abrasive material and/or thesubstrate being polished or buffed. The softer the substrate and theharder the abrasive material, the more it is likely that scratches ordefects may result.

A CMP method according to the present invention further includesremoving at least a majority portion of the flocculated portion of theabrasive material from the substrate. Such removal may be accomplishedby a variety of means and at a variety of points in a CMP method afterflocculation of abrasive material. Accordingly, in one aspect, brushscrubbing the substrate using a scrubbing solution comprising asurfactant material to flocculate abrasive material can remove at leasta majority portion of the abrasive material. Brush scrubbing may beperformed with a polyvinyl alcohol (PVA) brush. In another aspect,pressure spraying the substrate using a spray solution comprising asurfactant material to flocculate abrasive material can remove at leasta majority portion of the abrasive material. Flocculation may also beperformed by immersion in an aqueous bath comprising the surfactantmaterial. Of course, it is contemplated that flocculation and/or removalof abrasive material may occur at yet other points in a CMP method.High-pressure spray action may further be used to clean the flocculefrom the substrate.

EXAMPLE

Aqueous dilutions of the thirteen surfactants listed in Table 1 wereprepared at 0.1 wt % concentration using a heat/stir plate andde-ionized water. As mixed, Hitachi shallow trench isolation (STI)slurry containing a ceria abrasive material for CMP was added atapproximately 1 wt % loading to form an aqueous dilution of the slurrywith each of the thirteen surfactants. The Hitachi STI slurry was alsoadded to de-ionized water at approximately 1 wt % loading as a control.An additional thirteen slurry dilutions with surfactant and onede-ionized water control were prepared using aged, mixed Rodel tungstenCMP (WCMP) slurry containing alumina abrasive material, instead of theHitachi slurry. Approximately 15 hours after initial preparation, thedilutions were agitated to simulate initial conditions and anapproximately 1 ml sample of the supernatant of each solution wasdiluted 100 to 1 in de-ionized water. The diluted sample was analyzed byICP-OES with a Varian Liberty 110 unit using a standard V-groovenebulizer and polypropylene spray chamber. Emission spectra werereferenced to National Institute of Standards and Technology (NIST)traceable aluminum and cerium calibration standards. Samples were takenand analyses repeated each day for three days, on the seventh day, andon the thirteenth day following the initial agitation. Using the ICP-OESanalyses, ceria and alumina concentrations of the aqueous dilutions werecalculated and are presented, respectively, in Tables 2 and 3 below. Forsamples showing visible solids precipitation and an undetectable levelof metal content, suspended solids were estimated to be 0 wt %.

TABLE 1 Experimental Surfactants. Supplier Surfactant Description AirProducts CT-131 anionic/nonionic dispersant CT-141 anionic dispersantCT-324 wetting agent/dispersant Akzo Nobel Ethoquad 18/25 ethoxylatedstearyl methyl quaternary ammonium chloride Ethoquad C/25 ethoxylatedcocoalkyl methyl quaternary ammonium chloride Ashland Alkamuls PSMO-20ethoxylated (20) sorbitan monooleate Antarox P-104 ethoxylatedpolyoxypropylene Igepal DM-710 ethoxylated dinonylphenol/ nonophenol(branched) Rhodafac RE-610 polyoxyethylene (branched) nonylphenyl etherphosphate Rhodaquat cetyltrimethyl ammonium M-242B/99 bromide BYK-ChemieAnti Terra U80 unsaturated polyamine amide polymer acid salt, 2-butoxyethanol, xylene (dispersant blend) Uniqema Renex 30 ethoxylatedC₁₁-C₁₄ isoalcohols (wetting agent) Renex 36 ethoxylated C₁₁-C₁₄isoalcohols (wetting agent)

TABLE 2 % Ceria Results Surfactant 0 hrs. 26 hrs. 50 hrs. 75 hrs. 170hrs. 316 hrs. Control 1.11 0.85 0.65 0.63 0.56 0.51 CT-131 1.15 0.730.60 0.57 0.55 0.45 CT-141 1.09 0.51 0.52 0.47 0.45 0.40 CT-324 1.100.23 0.60 0.53 0.43 0.40 EQ-18/25 0.65 0.08 0.00 0.00 0.00 0.00 EQ-C/250.72 0.09 0.00 0.00 0.00 0.00 PSMO-20 1.10 0.83 0.61 0.59 0.52 0.47P-104 1.05 0.29 0.63 0.59 0.48 0.46 DM-710 1.07 0.36 0.56 0.49 0.43 0.42RE-610 0.86 0.33 0.25 0.22 0.24 0.18 M-242B/99 0.03 0.03 0.00 0.00 0.000.00 AT-U80 1.10 0.35 0.44 0.40 0.33 0.30 RX-30 1.11 0.70 0.60 0.49 0.430.40 RX-36 1.08 0.65 0.59 0.55 0.42 0.40

TABLE 3 % Alumina Results Surfactant 0 hrs. 27 hrs. 51 hrs. 75 hrs. 170hrs. 317 hrs. Control 1.09 0.14 0.12 0.13 0.00 0.00 CT-131 1.16 0.500.41 0.32 0.25 0.19 CT-141 1.19 0.67 0.50 0.40 0.29 0.21 CT-324 1.110.08 0.07 0.07 0.00 0.00 EQ-18/25 1.04 0.56 0.36 0.28 0.21 0.14 EQ-C/251.04 0.38 0.20 0.18 0.00 0.00 PSMO-20 1.16 0.18 0.15 0.15 0.00 0.00P-104 1.10 0.20 0.16 0.16 0.00 0.00 DM-710 1.23 0.18 0.15 0.15 0.00 0.00RE-610 1.15 0.94 0.82 0.76 0.59 0.55 M-242B/99 1.06 0.65 0.48 0.40 0.250.20 AT-U80 0.61 0.38 0.31 0.26 0.21 0.15 RX-30 1.10 0.16 0.15 0.14 0.000.00 RX-36 1.18 0.86 0.69 0.40 0.39 0.28

The results of ceria settling for the Hitachi STI slurry are summarizedin Table 2. The ceria control dilution showed minimal settling relativeto all experimental solutions over the thirteen day period. Such may bedue to the ingredients of the Hitachi STI slurry that encouragedispersal of ceria abrasive material and, thus, discourage settling.Notably, some surfactants caused ceria to settle much faster than thecontrol. In particular, both Ethoquads (EQ-18/25 and EQ-C/25) andRhodaquat (M−242B/99) caused such rapid settling that agitation did notrestore the original (as-prepared) ceria concentration. Accordingly, theceria concentration indicated for time 0 is much less than theapproximate 1 wt % loading of the other dilutions. These threesurfactants are cationic and are the only quaternary ammonium saltsamong the thirteen surfactants.

Some of the dilutions showed the strange effect of initially dropping inceria concentration, then increasing in concentration, and then droppingagain. This is typical of oscillating chemical reactions governed by thekinetics of two competing equilibria. Some other solutions showedminimal difference in settling as compared to the control. Only thethree quaternary ammonium salts produced complete settling of ceria bycompletion of the thirteen day trial.

The results of alumina settling are summarized in Table 3. The aluminain the Rodel WCMP slurry dilution without a surfactant (the controldilution) settled quickly. Approximately 90% of the initial aluminasettled within 24 hours. Most of the surfactants had little effect onalumina settling compared to the control dilution, suggesting minimalinteraction. Some surfactants showed moderate dispersion of alumina,reducing the degree of settling compared to the control dilution with20-30 wt % of the initial alumina remaining after thirteen days. Onesurfactant, Rhodafac (RE-610), provided enough dispersion to keepapproximately 50% of the original alumina dispersed after thirteen days.Rhodafac is an anionic surfactant and the only phosphate among thethirteen surfactants.

Using setup conditions and the first three days of settling data, firstorder settling rate constants were calculated using the equation setforth above and a one-hour settling rate constant was determined foreach surfactant as applied to both ceria and alumina. The one-hoursettling rate constants are listed in Table 4 below. Notably, the highersettling rate constants correspond to higher settling rates. Asexpected, the two Ethoquads and Rhodaquat exhibited the highest settlingrate constants for ceria abrasive material.

TABLE 4 Settling Rate Constants (1-hour) Surfactant Ceria AluminaControl 0.010 0.051 PSMO-20 0.011 0.046 CT-131 0.013 0.024 RX-30 0.0140.048 RX-36 0.015 0.013 CT-141 0.020 0.018 DM-710 0.023 0.046 P-1040.025 0.044 AT-U80 0.026 0.030 CT-324 0.028 0.065 RE-610 0.033 0.008EQ-C/25 0.098 0.035 EQ-18/25 0.102 0.024 M-242B/99 0.136 0.018

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A CMP method comprising applying a CMP slurry containingsubstantially dispersed, solid abrasive material to a substrate;applying to the substrate a surfactant containing material that exhibitsthe characteristic of decreasing a settling time for the abrasivematerial in an aqueous dilution of the slurry by complexing between atleast a portion of the abrasive material and the surfactant to formfloccule and flocculating at least a majority portion of the abrasivematerial with the surfactant; and removing at least some of the abrasivematerial.
 2. The CMP method of claim 1 further comprising polishing thesubstrate with the slurry before applying the surfactant containingmaterial.
 3. The CMP method of claim 1 wherein the abrasive materialcomprises ceria.
 4. The CMP method of claim 1 wherein the surfactantcomprises a cationic surfactant.
 5. The CMP method of claim 4 whereinthe cationic surfactant comprises a quaternary ammonium substitutedsalt.
 6. A CMP method comprising applying a CMP slurry containingsubstantially dispersed, solid abrasive material to a substrate;applying to the substrate a surfactant containing material, thesurfactant exhibiting a one-hour settling rate constant of greater than0.035 for the abrasive material in an aqueous mixture of about 0.1weight percent surfactant and about 1 weight percent slurry complexingbetween at least a portion of the abrasive material and the surfactantto form floccule; and flocculating at least a majority portion of theabrasive material with the surfactant and removing at least some of theabrasive material.
 7. The CMP method of claim 6 further comprisingpolishing the substrate with the slurry before applying the surfactantcontaining material.
 8. The CMP method of claim 6 wherein the abrasivematerial comprises ceria.
 9. The CMP method of claim 6 wherein thesettling rate constant is greater than about 0.09.
 10. The CMP method ofclaim 6 wherein the surfactant comprises a cationic surfactant.
 11. TheCMP method of claim 10 wherein the cationic surfactant comprises aquaternary ammonium substituted salt.
 12. A CMP method comprising:applying a CMP slurry containing a solid, ceria-comprising abrasivematerial to a substrate; polishing the substrate with the abrasivematerial; applying to the substrate a quaternary ammonium substitutedsalt surfactant containing material that exhibits the characteristic ofdecreasing a settling time for the abrasive material in an aqueousdilution of the slurry; flocculating and complexing the abrasivematerial with the surfactant; and removing at least a majority portionof the abrasive material.
 13. The CMP method of claim 12 wherein atemperature of the substrate during the flocculating does not exceedabout 40 degrees Celsius.
 14. The CMP method of claim 12 wherein thequaternary ammonium substituted salt exhibits a one-hour settling rateconstant of greater than 0.09 for the abrasive material in an aqueousmixture of about 0.1 weight percent surfactant and about 1 weightpercent slurry.
 15. The CMP method of claim 12 wherein a concentrationof the quaternary ammonium substituted salt in the surfactant containingmaterial is from about 10 micrograms per milliliter (μg/ml) to about10,000 μg/ml.
 16. The CMP method of claim 12 wherein the quaternaryammonium substituted salt comprises a quaternary ammonium halide. 17.The CMP method of claim 16 wherein the quaternary ammonium halidecomprises a cetyltrimethylammonium bromide.
 18. The CMP method of claim16 wherein the quaternary ammonium halide comprises a polyethoxylatedquaternary ammonium halide.
 19. A CMP method comprising: applying a CMPslurry containing substantially dispersed, solid abrasive ceria to asubstrate; applying to the substrate a surfactant containing materialthat exhibits the characteristic of decreasing a settling time for theabrasive ceria in an aqueous dilution of the slurry by complexingbetween at least a portion of the abrasive ceria and the surfactant toform floccule; and removing at least some of the floccule containingabrasive ceria.